1. Field of the Invention
The present invention relates to lithography systems, specifically correcting ellipticity and field uniformity in the illumination beams of lithography scanners.
2. Background Art
Conventional lithography systems include, among other things, an illumination system that produces an illumination beam for exposing a substrate via a patterned reticle. The quality of the illumination is defined by the uniformity of the illuminated field (i.e., the total amount of energy at any point in the field), the uniformity of the pupil (i.e., the energy distribution in the pupil), and the consistency of the energy distribution in the pupils across the field. To quantitatively characterize the energy distribution in the pupil, a generalized parameter called ellipticity is used. FIGS. 7A and 7B further illustrate the concept of ellipticity.
FIG. 7A is an illustration of an ideal beam of light 702 that is focused on a point 704 on a reticle plane 706. Pupil 708 is a cross-section of beam 702, which represents the pupil at a defocus position of the beam. FIG. 7B is a front view of pupil 708. Pupil 708 is annular and has four quadrants. In an ideal beam with no ellipticity, such as beam 702, the energy E is uniformly distributed between all four quadrants. Ellipticity occurs when the energy distribution in the quadrants becomes unbalanced. Specifically, ellipticity may be defined as:
                              Ellipticity          =                      100            ⁢            %            ⁢                                                  ⁢                          (                              1                -                                                                            E                      1                                        +                                          E                      2                                                                                                  E                      3                                        +                                          E                      4                                                                                  )                                      ,                            (                  Eq          .                                          ⁢          1                )            where E1, E2, E3, and E4 are the energies in each of the respective quadrants (as illustrated by the shaded areas in FIG. 7B).
Ellipticity can affect the degree of exposure of a substrate which, in turn, can cause variations in linewidth dimensions of lithographic patterns and resulting electronic elements formed on the substrate. Where these variations in linewidth dimensions are such that there is a difference between linewidth dimensions for horizontal lines and linewidth dimensions for vertical lines, the condition is referred to as horizontal-vertical (H-V) bias. Because H-V bias can effect the performance of an integrated circuit, methods for improving the control of variations in linewidth dimensions have been the subject of assorted efforts.
One example of a popular conventional lithography system is a step and scan system (sometimes referred to as a scanner). A step and scan system includes an illuminated slit narrower than one exposure field. The system then scans the reticle and wafer synchronously by the slot to expose each field on the wafer. This process is repeated. Because of the nature of the system's operation, radiation energy in the scan direction is integrated, and as a result a dose received on the photo-active coating on the substrate can be non-uniform. Non-uniformity in the dose can cause printing errors and degraded device performance.
Some lithography systems use uniformity correction systems to make the scan-integrated intensity profile uniform. Some of these uniformity correction systems also attempt to correct for ellipticity in the illumination beam. However, these existing systems can only correct average ellipticity across the field. They cannot correct for one or more ellipticity variations across the field. Additionally, when uniformity correction systems are also used to correct ellipticity, differences in trends between ellipticity and uniformity are unaccounted for. For example, beam intensity at the edges of the illumination field in the cross scan direction might be higher than in the center, causing the uniformity correction system to attenuate light only at shallow edges of the field for uniformity correction. At the same time, ellipticity may be higher in the center of the field, meaning that light attenuation would need to extend deeper into the center of the field. Existing systems cannot fulfill both of these functions at once.
What is needed is a system and method for reducing ellipticity in an illumination beam in a manner that is independent from other features of the illumination beam, and that accounts for variations in the angular distribution of light in the illumination beam across an illumination field.